Metal vapor discharge lamps of high luminance, high efficiency and long life, such as a metal halide lamp (hereinafter, simply referred to as “lamp”), have been widely used in many places for the above features.
A conventional lighting fixture that uses the above lamp as a light source further includes, in addition to the lamp, a reflector that has a concave reflecting surface that reflects light emitted from the lamp in a desired direction. The reflector's light output opening is covered with, for example, a front glass plate (what is called a closed type lighting fixture). Note that the reason for covering the light output opening is to prevent broken pieces of the lamp from scattering outside of the lighting fixture when the lamp (arc tube) is broken for some reasons.
Recently, there has been a demand for a so-called open type lighting fixture whose light output opening is not covered with a front glass. To meet such a demand, a lamp with the following structure is suggested. For example, the lamp is composed of an arc tube, an inner tube and an outer tube, which is so-called a triple-tube structure. The inner tube houses the arc tube, and the outer tube houses the inner tube. If the inner tube is damaged because of the breakage of the arc tube, broken pieces of the inner tube remain within the outer tube. (e.g. Patent Document 1).
Patent Document 1: Japan Unexamined Patent Publication No. H11-96973